PARIS—The big surprise with Pope Benedict’s new book is not that he believes the Catholic Church can permit condom use to prevent the spread of AIDS in some circumstances, but that he took so long to say so.

Quotes from a new book of interviews with him made headlines around the world and some commentators went overboard by saying the Roman Catholic Church had made a sudden about-face on birth control and finally caught up with modern society.

A close reading of those quotes shows the pontiff not breaking from past teachings but thinking his way through the issue with logic dating back to the 13th century Saint Thomas Aquinas. He concludes that condom use, while still wrong, can be a lesser evil in certain circumstances.

Many Catholic theologians came to the same conclusion years ago and some priests in Africa privately advise this if the alternative is infection, for example to a woman whose HIV-positive husband demands sex.

But this is the first time a pope has publicly said it. The issue has been a minefield for popes and successive attempts to explain Church policy have backfired, as Benedict himself found out on a visit to Africa last year.

Back then, he caused an international uproar when he told journalists accompanying him to Africa that condoms should not be used because they could worsen the spread of AIDS.

The pontiff has now turned to a trusted Catholic journalist, fellow Bavarian Peter Seewald whose long interview with the pope forms the new book, to help reframe the argument in a way that makes Church doctrine seems less cold and absolute.

“The Church needed to get this clarification out in a way that was not a major document,” said British Catholic journalist Austen Ivereigh. “The pope used the informal format of a book interview to signal what seems to be a major shift but is no more than an expression of the obvious.”

The Vatican convened a commission of moral theologians to study the issue in 2006 but their report, which Vatican sources say echoed what Benedict has said, was quietly shelved out of fear that any public statement would be misunderstood.

In the book “Light of the World,” Benedict stresses condom use is not a morally acceptable solution to the AIDS epidemic because he says it can turn sex from an expression of selfless love to “a sort of drug that people administer to themselves.”

He then says it may be justified in some cases, such as that of an infected male prostitute protecting his sex partner.

“This is a first step in the direction of a moralisation, a first assumption of responsibility,” he said, “but it is not really the way to deal with the evil of HIV infection.”

But while condom use does not block procreation in gay sex, it does do so in sex between men and women. Benedict stressed this in the book by repeating his support for Humanae Vitae, the 1968 encyclical banning artificial birth control.

The problem for the Church is that it takes only a small step in logic to go from using this argument for gay men to using it for heterosexuals, or widening it to other issues.

“The main consequence is that the Church can now talk about this issue with greater credibility,” Ivereigh said. “People have just shut off listening to the Church on this subject. They assume it has a dogmatic and inhumane stance.